Theming design spec: Tree view

[phcox]

Create a detailed theming design spec for the Tree view component under the Unified theme initiative. Reference epic #1513. Include updated tokens, icons, interactions, new variants, and before/after views.

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Jira Issue: PF-3400

[evwilkin]

Closed via Jira sync - Jira issue PF-3400 was closed.

[patternfly-jira-sync]

Closed via Jira sync - Jira issue PF-3400 was closed.